In Long Term Evolution-Advanced (LTE-A), which is the next-generation cellular communication standard that is discussed in Third Generation Partnership Project (3GPP), introduction of technology called carrier aggregation (CA) has been studied. The carrier aggregation is technology that forms a communication channel between a user equipment (UE) and a base station (BS, or evolved Node B (eNB)) by aggregating a plurality of frequency bands that are supported in LTE, for example, and thereby improves communication throughput. Each frequency band included in one communication channel by the carrier aggregation is called a component carrier (CC). The bandwidths of frequency bands that are available in LTE are 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20 MHz. Accordingly, if five bands of 20 MHz are aggregated as component carriers, a communication channel of 100 MHz in total can be formed.
Component carriers that are included in one communication channel in the carrier aggregation are not necessarily contiguous to one another in the frequency direction. The mode in which component carriers are arranged contiguous to one another in the frequency direction is called a contiguous mode. On the other hand, the mode in which component carriers are arranged not contiguous to one another is called a non-contiguous mode.
Further, in the carrier aggregation, the number of component carriers in an uplink and the number of component carriers in a downlink are not necessarily equal. The mode in which the number of component carriers in an uplink and the number of component carriers in a downlink are equal is called a symmetric mode. On the other hand, the mode in which the number of component carriers in an uplink and the number of component carriers in a downlink are not equal is called an asymmetric mode. For example, in the case of using two component carriers in an uplink and three component carriers in a downlink, it is asymmetric carrier aggregation.
In radio communication of a conventional cellular system involving the carrier aggregation, allocation information of communication resources (that is, scheduling information) is transmitted from a base station from each user equipment over a downlink control channel (for example, PDCCH: Physical Downlink Control Channel). In radio communication not involving the carrier aggregation, the scheduling information can be transmitted according to two kinds of methods. A first method is a method in which a conventional method is applied to individual component carriers as is. In other words, in the first method, a control channel for transmitting scheduling information on data transmission in a certain component carrier is set inside the corresponding component carrier. In this disclosure, the first method is referred to as a straight scheduling method. On the other hand, in a second method, a control channel for transmitting scheduling information on data transmission in a certain component carrier is set inside a component carrier different from the corresponding component carrier. According to the second method, when communication resources used for transmission of scheduling information are aggregated, a ratio of overhead occupied by communication resources is reduced. Thus, the second method can achieve a higher throughput than the first method. In this disclosure, the second method is referred to as a “cross scheduling method.” The straight scheduling method and the cross scheduling method can be complexly used within one communication channel. In other words, one channel configuring a communication channel may be operated by the cross scheduling method, and another channel may be operated by the straight scheduling method.
A handover, which is a basic technique for achieving the mobility of a user equipment in the cellular communication standard, is one of important subjects in LTE-A. In LTE, a user equipment measures a communication quality over a channel with a serving base station (a currently connected base station) and communication qualities with peripheral base stations and transmits a measurement report containing measurements to the serving base station. Receiving the measurement report, the serving base station determines whether to execute a handover based on the measurements contained in the report. Then, if it is determined that a handover is to be executed, a handover is carried out among a source base station (the serving base station before a handover), the user equipment, and a target base station (a serving base station after a handover) in accordance with a prescribed procedure (e.g. cf. Patent Literature 1 below).